1. Field of the Invention
This invention relates to high-performance hearing protection or communications earcups and seals, more particularly to hearing protection or communications earcups and soft seals that may be manufactured with a minimum number of operations while providing excellent acoustic protection as well as a comfortable contact surface for the user.
2. Description of Related Art
Earcups are widely used in both industry and in the military. Recently, Poiesis Research, Inc., of Pensacola, Fla., working with the U. S. Navy Aerospace Medical Research Laboratory at Pensacola, Fla., demonstrated an advanced passive earcup and seal design with over 45 dB of attenuation at 100 Hz. This earcup was made possible by advances in materials science described in U.S. Pat. No. 5,400,296 issued to Cushman, et al. As it is generally acknowledged within the hearing protection field that any more than 50 dB of attenuation would be masked by bone conduction and, therefore, superfluous, this earcup and seal come very close to being "perfect." These test results were, however, obtained in a laboratory situation and may not reflect conditions in the real world. The earcup seal was placed against a polished stainless-steel flat-plate coupler that had been lightly coated with vacuum grease to preclude gas leaks at the contact surface. It is impractical to assume that users of any earcup design will be willing to apply a coat of grease to insure a gas-tight seal against their uneven head. These laboratory data show that under ideal conditions an earcup and seal can perform very well indeed and suggest that the earcup part of the combination requires little improvement in terms of performance. The soft seal between the earcup and the user's head can be improved considerably, however, and improvements in the manufacturability of both the earcup and the soft seal can be realized.
Some form of soft-seal is necessary in any earcup design to allow the earcup and seal to accommodate to the idiosyncrasies of individual head contours. Currently marketed earcup seals fall generally into two categories: those using an envelope filled with an open-celled foam and those using an envelope containing a liquid or gel. Earcup seal envelopes are usually made from vinyl or urethane film. In all cases the seal envelope has a characteristic acoustic impedance that differs from air and will reflect a great percentage of the impinging energy because of this impedance difference.
The characteristic acoustic impedance for a particular material is calculated by multiplying the density of the material times the speed of sound within the material, which results in impedance, Z, in Rayls. Characteristic acoustic impedance mismatches always cause some portion of the impinging acoustic pressure to be reflected, thus attenuating that portion transmitted past the mismatched boundary. The ratio of acoustic reflections at any interface between two media can be calculated if the characteristic acoustic impedances of the two materials is known using the following formula, taken from Erwin Meyer and Ernst-Georg Neumann in their text Physical and Applied Acoustics, an Introduction, Academic Press, 1972, Section 1.4, transmission line theory. The impedances of water and air are used to illustrate. ##EQU1## Water at a boundary with air is a very good acoustic reflector.
An earcup seal made from a vinyl envelope containing a liquid would have impedance mismatches at the outer surface between air and vinyl; the inner surface of the envelope between vinyl and the liquid; the inner surface of the envelope between the liquid and vinyl; and, finally, the outer surface of the envelope between vinyl and air. Of course, the magnitude of the impedance mismatch between vinyl and a liquid is much less than between vinyl and air. A similar impedance mismatch analysis can be made for a foam-filled earcup seal within a vinyl envelope.
Open-celled foams are known acoustic attenuators at high frequencies. This is because acoustic pressure forces air to pass through small pores in open cell foam where the air flow becomes turbulent and gives rise to viscous damping. As the wavelength becomes long relative to the thickness of foam used, however, the slope of the pressure differential becomes shallow and viscous damping is diminished. For viscous damping to be maximally effective there should be at least 1/2 wavelength present within the foam.
Sound travels at roughly 344 meters per second in air. A foam sample thickness of 2.5 centimeters will lose effectiveness for wavelengths longer than about 5 centimeters. That is, a foam sample thickness of 2.5 centimeters will lose effectiveness for frequencies below about 6880 Hertz. To be effective down to the lower-end frequency threshold for human hearing, at about 20 Hertz, an open-cell foam type of earcup seal would have to be 8.6 meters thick. These data suggest that an earcup seal containing an open-celled foam would perform extremely poorly, and earcup seals of this design do not. However, experience shows that if the envelope surrounding the open-celled foam is removed performance is extremely poor. If the reverse experiment is made, and the envelope alone is used, performance is also extremely poor. These results arise because it is the interaction of the foam with the seal envelope that is producing damping. Even though the wavelength of impinging acoustic energy may be very long relative to the envelope-to-foam interface, the envelope film is thin and has little inertia so it will be "pumped" diaphragmatically by impinging energy, thus resulting in cyclical "rubbing" of the envelope film against the foam interface to produce slip damping.
Liquid or gel-filled earcup seals perform in a similar fashion. As the envelope film is "pumped" by cyclical acoustic pressures viscous damping at the interface between the envelope membrane and the liquid or gel takes place. Liquids and gels themselves are excellent conductors of acoustic energy.
In light of the above discussion it should be clear that adding more liquid or more foam or both will not substantially improve the performance of an earcup seal, but adding more impedance mismatched layers perpendicular to acoustic pathways most certainly will. Aileo exploited an impedance mismatching principle in his U.S. Pat. No. 3,506,980 in which a series of concentric rings and interleaving air form an earcup seal with multiple impedance mismatches at each ring surface. Adding more impedance mismatched layers in slip damping contact with foam or fiber inserts or gel or liquid filled chambers is an even better approach that has been taken with the instant invention.
In the design of an earcup and seal consideration must also be given to how an earcup seal with make contact with both the earcup and the person using it. Air leaks between the seal and earcup or between the seal and the head of the person using it are good pathways for acoustic energy. Both pathways degrade performance significantly. Finally, any design must be manufacturable to be practical, and a minimum number of injection moldable parts is distinctly desirable, as is ease of assembly.